3GPP LTE (3rd Generation Partnership Project Long-term Evolution) has adopted a CG (computer generated) sequence for transmission bandwidths of 1 and 2 RBs and a ZC (Zadoff-Chu) sequence for transmission bandwidths of 3 RBs or more as code sequences used for data demodulation reference signals (DM-RS: demodulation-reference signal) used in uplink.
In an LTE uplink, a large number of sequences are divided into 30 sequence groups in each transmission bandwidth (1 to 110 RBs (resource blocks)) and a transmission bandwidth (more specifically, the number of allocated RBs) is associated with a sequence in each sequence group. Each sequence group is assigned a different number (sequence group number) and each cell is assigned one sequence group as shown in FIG. 1 (cell-specific sequence group assignment). Since each sequence group is configured of sequences having a high cross-correlation among sequences, the probability that sequences with a high cross-correlation may be used among neighboring cells is reduced and interference among neighboring cells is thus reduced. Since a base station (may also be called “eNB”) broadcasts a sequence group number to a terminal (may also be called “UE (User Equipment)”) in the cell, even when a transmission bandwidth is changed, the terminal can identify a DM-RS code sequence number, which in turn makes it possible to omit signaling of the code sequence number.
The reason that a CG sequence which is different from a ZC sequence is selected in transmission bandwidths of 1 and 2 RBs is that the ZC sequence for transmission bandwidths of 1 and 2 RBs cannot secure 30 (number of sequence groups) sequences. That is, since 30 ZC sequences cannot be secured in transmission bandwidths of 1 and 2 RBs, a reuse factor of sequence (the number of repetitions of a ZC sequence number used as a DM-RS) decreases, so that the same sequence number may be assigned in neighboring cells, thus increasing inter-cell interference. Thus, the CG sequence is adopted in transmission bandwidths of 1 and 2RB.
Since the sequence length of a ZC sequence is a prime number, for a DM-RS code sequence (ZC sequence) of 3 RBs or more, a sequence is used in which the leading portion of the ZC sequence which is smaller than a DM-RS in size is copied (extended) to the rear end portion (e.g., see FIG. 2).
The ZC sequence is a kind of CAZAC (constant amplitude and zero auto-correlation code) sequence and expressed by following equation 1. The number of ZC sequences generated is (ZC sequence length NZCRS−1).
                    [        1        ]                                                                                                                                  r                  _                                                  u                  ,                  v                                            ⁡                              (                n                )                                      =                                          x                q                            ⁡                              (                                  n                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                                      N                    ZC                    RS                                                  )                                              ,                      0            ≤            n            <                          M              sc              RS                                      ⁢                                  ⁢                                                            x                q                            ⁡                              (                m                )                                      =                          ⅇ                                                -                  j                                ⁢                                                                  ⁢                                                      π                    ⁢                                                                                  ⁢                                          qm                      ⁡                                              (                                                  m                          +                          1                                                )                                                                                                  N                    ZC                    RS                                                                                ,                      0            ≤            m            ≤                                          N                ZC                RS                            -              1                                                          (                  Equation          ⁢                                          ⁢          1                )            
ZC sequence number q assigned to each sequence group is calculated by equation 2.[2]q=└ q+½┘+v·(−1)└2 q┘ q=NZCRS·(u+1)/31  (Equation 2)
Note that as shown in equation 3, sequence group number u is associated with cell ID(NIDcell) and sequence group hopping is applied in order to randomize interference between sequence groups.
                                              ⁢                  [          3          ]                                                                                              ⁢                              u            =                                          (                                                                            f                      gh                                        ⁡                                          (                                              n                        s                                            )                                                        +                                      f                    ss                                                  )                            ⁢              mod              ⁢                                                          ⁢              30                                ⁢                                          ⁢                                                    f                gh                            ⁡                              (                                  n                  s                                )                                      =                          {                                                                                                                  0                                                                                              if                          ⁢                                                                                                          ⁢                          group                          ⁢                                                                                                          ⁢                          hopping                          ⁢                                                                                                          ⁢                          is                          ⁢                                                                                                          ⁢                          disabled                                                                                                                                                                                          (                                                                                          ∑                                                                  i                                  =                                  0                                                                7                                                            ⁢                                                                                                c                                  ⁡                                                                      (                                                                                                                  8                                        ⁢                                                                                  n                                          s                                                                                                                    +                                      i                                                                        )                                                                                                  ·                                                                  2                                  i                                                                                                                      )                                                    ⁢                          mod                          ⁢                                                                                                          ⁢                          30                                                                                                                      if                          ⁢                                                                                                          ⁢                          group                          ⁢                                                                                                          ⁢                          hopping                          ⁢                                                                                                          ⁢                          is                          ⁢                                                                                                          ⁢                          enabled                                                                                                      ⁢                                                                          ⁢                                                                          ⁢                                      f                    ss                    PUCCH                                                  =                                                                            N                      ID                      cell                                        ⁢                    mod                    ⁢                                                                                  ⁢                    30                    ⁢                                                                                  ⁢                                                                                  ⁢                                          f                      ss                      PUSCH                                                        =                                                            (                                                                        f                          ss                          PUCCH                                                +                                                  Δ                          ss                                                                    )                                        ⁢                    mod                    ⁢                                                                                  ⁢                    30                                                                                                          (                  Equation          ⁢                                          ⁢          3                )            
Using equation 2, a combination of sequences whose q/NZCRS value which is a ratio of ZC sequence number q and ZC sequence length NZCRS becomes closer to each other is assigned to an identical sequence group. Sequences whose inter-sequence q/NZCRS becomes closer to each other means sequences with a high cross-correlation. It is thereby possible to reduce the cross-correlation between sequence groups (e.g., see PTL 1).
A CG sequence is expressed, for example, by following equation 4 (e.g., see NPL 1). Note that, φ(n) shown in equation 4 is defined in each sequence group number u for a transmission bandwidth of 1 RB as shown in FIG. 3 and defined in each sequence group number u for a transmission bandwidth of 2 RBs as shown in FIG. 4 (e.g., see NPL 1).[4] ru,v(n)=ejφ(n)π/4,0≦n≦MscRS−1  (Equation 4)
In LTE-Advanced which is an evolved version of LTE, studies are being carried out on a heterogeneous network (HetNet) using a plurality of base stations having coverage areas different in size for further capacity improvement. For example, in an operation of HetNet, a picocell (LPN (low power node), also called “small power RRH” (remote radio head)) having small transmission power is disposed within a coverage area of a macrocell (also called “HPN” (high power node)) having large transmission power.
In LTE-Advanced, studies are further being carried out on application of CoMP (coordinated multiple point transmission and reception) which is a communication scheme whereby a plurality of cells (base stations) cooperate to transmit/receive signals to/from a terminal for the purpose of improving throughput of users mainly located on a cell edge in a HetNet environment. For example, in the case of UL (uplink) CoMP, a plurality of cells (base stations) cooperate to receive signals from one terminal, then combine the signals and thereby improve receiving quality. In order to reduce the influence of inter-cell interference, a plurality of cells cooperate to schedule terminals to prevent inter-cell interference from increasing within a CoMP set (group of cells that cooperate to perform transmission/reception).
Studies are being carried out on introduction of MU-MIMO (multiple user-multiple input multiple output) communication of a plurality of terminals to which UL CoMP is applied to obtain more performance improvement effects of the above-described UL CoMP, that is, terminals (hereinafter may also be referred to as “UL CoMP terminals”) that cooperate to receive and combine transmission signals. MIMO communication is a technique that provides the transmitting and receiving sides with a plurality of antennas and enables simultaneous spatially multiplexed transmission of different signal sequences at the same frequency. MU-MIMO communication is a technique of carrying out MIMO communication between a plurality of terminals and a base station, and can improve frequency utilization efficiency of a system.
To demultiplex signals from different terminals in MU-MIMO communication, it is necessary to transmit DM-RSs which are orthogonal between the terminals. There are various methods of orthogonalizing DM-RSs such as application of OCC (orthogonal cover code) or use of a ZC sequence with CSs (cyclic shifts) differing among terminals applied (CS-ZC sequence). When OCC is applied, DM-RSs for a maximum of two terminals (UEs) can be orthogonalized. A CS-ZC sequence is a sequence obtained by cyclically shifting a ZC sequence. In the CS-ZC sequence, by setting a greater value than a maximum propagation delay time of a transmission signal of a terminal as an amount of cyclic shift, it is possible to orthogonalize a plurality of CS-ZC sequences obtained by cyclically shifting a ZC sequence of the same sequence number.
However, since DM-RSs are received by a plurality of different cells in UL CoMP, there are cases with the aforementioned cell-specific sequence group assignment, where sequence numbers used vary among terminals for which MU-MIMO communication is introduced, causing a situation where DM-RSs cannot be orthogonalized. For this reason, as shown in FIG. 5, studies are being carried out on introducing, for terminals to which CoMP is applied, a UE-specific sequence (Group#2 in FIG. 5) in which a sequence is indicated to each terminal instead of using a cell-specific sequence (Group#0,#1 in FIG. 5), and orthogonalizing DM-RSs between UL CoMP terminals.
MU-MIMO communication involves a problem in that, when transmission bandwidths vary among multiplexing target terminals, CS-ZC sequences used among the terminals (CS-ZC sequences of different bandwidths) are not orthogonalized.
To solve the problem that CS-ZC sequences of different bandwidths are not orthogonalized, NPL 2 discloses that it is possible to orthogonalize CS-ZC sequences of different transmission bandwidths within an identical sequence group (combination of sequences having close q/NZCRS) by applying, to the ZC sequences, cyclic shift (CS) corresponding to the transmission bandwidth. This makes it possible to increase the number of terminals that can orthogonalize DM-RSs in MU-MIMO communication and increase the multiplex number of MU-MIMO communication.